A damper operates in vehicle suspensions as a damping device controlling the sprung (body) and unsprung (wheels) masses of a vehicle by reducing loads or vertical accelerations normally transmitted from the wheels to the body. Damping is accomplished by converting kinetic energy into thermal energy and dissipating the heat. Conventionally, hydraulic dampers include a piston with a connected piston rod slidably carried in a fluid-filled tube and separating the tube into extension and compression chambers. A rod guide at the top end of the tube closes the extension chamber and slidably engages the piston rod. As the parts of the vehicle suspension to which the cylinder tube and piston rod are attached move relative to one another, the damping piston assembly is moved in compression and rebound strokes along the axis of the damper. The damping piston assembly includes passages, and in some dampers also special valve arrangements associated with the passages, that allow fluid in the working chamber to flow through the damping piston assembly at a controlled rate to provide damping of the relative motion between the parts of the vehicle suspension to which the damper is attached.
In many applications, the suspension damper is called upon to limit the full extension travel of the suspension system. In some prior dampers, mechanical rebound stops that are fixed to the piston rod and engageable with the rod guide are known to provide a means of limiting the maximum extension travel of the piston rod from the damper. A typical mechanical rebound stop is generally equipped with a resilient bumper made of material such as rubber or urethane. The bumper is designed to cushion the engagement of the damping piston with the rod guide at the end of damper travel in the extension direction. This type of a mechanical stop tends to result in somewhat of an abrupt means of limiting travel during rebound. It has also been found that, in severe applications, a resilient bumper material may undesirably experience heat degradation when the bumper absorbs the entire rebound stop load.
It has also been the practice in some prior hydraulic dampers to provide elements attached to the damping piston assembly and the cylinder tube that provide additional hydraulic damping force acting against the piston during a portion of the rebound stroke, for slowing the damping piston assembly as it approaches the end of the rebound stroke. This function of providing additional damping at the end of the rebound stroke, for slowing the rate of rebound, is also known as hydraulic “rebound cut-off.” Examples of this approach are disclosed in U.S. Pat. No. 6,209,691 to Fehring et al. and U.S. Pat. No. 5,706,920 to Pees et al., and in British Patent No. 691,477 to Stephens.
In recent years, hydraulic dampers using a special type of fluid, known as Magneto-Rheological (MR) fluid, have been utilized as part of vehicle traction and stability enhancement control systems for actively controlling the amount of damping provided under varying road and operating conditions to provide improved performance and safe operation of vehicles. An MR fluid is generally significantly more viscous and has a higher specific gravity than the hydraulic fluids used in prior vehicle dampers. As a result, elements for providing a hydraulic rebound cut-off function in prior hydraulic dampers, or spacers attached to the damping piston assembly and/or piston rod for limiting maximum extension or speed of extension of the damper on the rebound stroke, may provide inefficient and undesirable performance in dampers using MR fluids or other fluids.
Providing a hydraulic rebound cut-off feature with a shock absorber form of damper is known. Such a device is disclosed in U.S. Pat. No. 2,379,750. That hydraulic rebound cut-off feature uses a rod guide having a collar forming an anchorage for an upper end of a coil spring whose lower end is secured to a ring valve. When the piston approaches full extension, the ring valve is contacted, which closes some fluid passages completely and others partially to reduce their fluid flow capacity, increasing damping force and slowing extension directed travel. This prior art device undesirably restricts fluid flow between the valve and the piston.
Yet another type of known hydraulic rebound cut-off feature utilizes a rebound cut-off piston in conjunction with a damping piston. Such a device is disclosed in U.S. Pat. No. 4,342,447. According to this prior art design, a fixed/clamped disk or disk stack on a secondary or rebound cut-off piston co-acts with the damping piston to effect a substantial entrapment of fluid in the extension chamber of the shock absorber as the damping piston approaches full rebound. However, in this device, an indentation in the wall acts as a piston stop and not as a support for the rebound cut-off piston.
Still another type of known hydraulic rebound cut-off feature utilizes a rebound cut-off device in conjunction with a damping piston. Such a device is disclosed in U.S. Pat. No. 5,277,284. According to this prior art design, a spring is held on the damping piston by a retaining ring on the piston rod. However, in this device, the retaining ring does not act as a support for the rebound cut-off device.
In other dampers, the rebound cut-off disk is retained by a snap ring in the wall of the tube, or is attached to the rod guide by means of a spring. However, the snap ring reduces available stroke, while the attachment is awkward and prone to failure. Still other dampers include a rebound cut-off disk that is heavier than fluid in the tube, and they are supported by a plastic float. The float requires a certain volume and, therefore, takes up valuable stroke length. Moreover, in a monotube strut, the reservoir is upside down, so the float will not stay near the rod guide.
Particularly with monotube design dampers, maximizing active length is critical. This is because a typical monotube damper carries a gas cup that separates out a gas chamber within the single tube of the device. The gas chamber is expansible and contractible to account for the changing volume of space occupied by the piston rod entering and exiting the tube. Presence of the gas chamber minimizes the amount of active length that can be utilized by other features such as the rebound cut-off device. Therefore, there is a need in the art for a rebound cut-off feature for a monotube damper with minimal impact on damper dead length.